1. Field of the Invention
The present invention relates to a process for the preparation of optical devices, such as sensors, gratings, splitters, couplers, and the like. More particularly, the present invention relates to a continuous or stepwise continuous process for making optical sensors which uses as a starting material optical fiber elements comprising an optical fiber coated with at least one thermally removable coating. In the process of the invention, all or a portion of the coating is thermally removed to sufficiently expose the bare optical fiber to allow subsequent processing into an optical fiber device. The thermal removal of the coating is performed under conditions such that the optical fiber substantially retains its physical integrity.
2. Description of Related Art
Glass optical fibers are particularly susceptible to chemical and/or mechanical attack, which greatly decreases the physical integrity of the optical fibers and leads to their premature failure. Therefore, in the construction of glass-based optical fiber elements, a coating is usually applied to a glass optical fiber immediately after drawing to protect the bare glass surface of the fiber from the detrimental effects of chemical and/or mechanical attack which would otherwise occur.
If the coated optical fiber element is to be used in the manufacture of an optical fiber device, it may be necessary to thermally, chemically or mechanically remove all or a part of the protective coating from the coated optical fiber to leave a bare fiber surface. The bare fiber or bare fiber section which remains may then be further processed to form an optical fiber sensor. See, for example, Rizvi and Gower, Production of Bragg Gratings in Optical Fibers by Holographic and Mask Production Methods, The Institute of Electrical Engineers, Optical Fiber Gratings and Their Applications, January 1995. However, conventional thermal, mechanical or chemical means for stripping the coating from the bare fiber in sensor manufacturing processes reduce the physical integrity of the fiber. For example, mechanical stripping with a knife or tool may cause scratches on the glass fiber surface, which ultimately lead to fine cracks and decreased fiber strength. Solvents or concentrated acids may be applied to the optical fiber element to swell the coating and facilitate its removal, but such chemical stripping techniques often leave a residue on the fiber surface which reduces fiber strength and interferes with subsequent processing steps. Heat may be applied to deteriorate or burn away the coating, but the charred residue which results reduces fiber strength and may require additional coating removal steps prior to processing. In addition, the glass fiber absorbs heat during coating pyrolysis, which may result in fiber embrittlement. See, e.g., M. C. Farries et al., Fabrication and Performance of Packaged Fiber Gratings for Telecommunications, The Institute of Electrical Engineers, Optical Fiber Gratings and Their Applications, January 1995; Tang et al., Annealing of Linear Birefringence in Single-Mode Fiber Coils: Application to Optical Fiber Current Sensors, Journal of Lightwave Technology, vol. 9, No 8, August 1991.
U.S. Pat. No. 4,957,343 to Sato et al. describes a method for splicing "plastic clad" optical fibers using fusion bonding with a high temperature electrical discharge. The splicing method in Sato is conducted using optical fibers with a glass core, a polymeric clad layer coated adjacent the core, and a protective sheath coated adjacent the clad layer. The Sato reference teaches that the clad layer adjacent the glass core be made of a resin which, when pyrolyzed in the high temperature electric arc during fusion bonding, leaves only a small residuum at the fiber endfaces. Sato et al. state that any coating may be used which has a residuum, following thermogravimetric analysis, less than a predetermined amount, preferably 10% or less by weight, more preferably 3% or less by weight. Materials suggested for the clad layer include fluorine-containing methacrylates and polyfluorovinylidene.
Sato et al. claim that, compared to splices formed at the endfaces of uncoated (air-clad) fibers, the residuum which remains at the endface following fusion splicing does not significantly increase splice losses in the fused fiber. Thus, reliable splices may be formed without the need for removal of the clad layer prior to splicing, and the integrity of the optical path is preserved. With respect to physical integrity, the high temperature fusion bonding procedure described in the '343 patent is claimed not to "deteriorate" the glass fiber (col. 2, lines 38-41). However, as noted above, it is known in the art that rapid heating of the glass fiber causes fiber embrittlement, and the working examples of the '343 patent state that reinforcement is required to increase strength following the splicing procedure (col. 5, lines 40-45). In addition, manufacture of many types of optical devices requires removal of a significant length, or the entirety, of the coating from an optical fiber. The high-temperature pyrolysis described in Sato et al. has not been demonstrated effective for removal of large lengths of coating without deterioration of the fiber's physical properties, and would not be expected to be practical for continuous or stepwise continuous coating removal operations on a commercial scale.
Thus, whether protective optical fiber coatings are partially or totally removed in sensor manufacturing processes, an unknown amount of surface damage will occur from the exposure and physical handling of the optical fiber during mechanical, chemical or thermal stripping operations. Accordingly, a need exists in the art for a commercially practicable continuous or stepwise continuous coating removal procedure which minimizes degradation of the fiber's physical properties and substantially preserves the pristine fiber surface to permit effective subsequent processing. The desired process would reduce or eliminate fiber handling steps and, where applicable, minimize the exposure time of the bare fiber before recoating.